1. Field of the Invention
The present invention relates generally to a two-wheel vehicle structure, more particularly, relates to a two-wheel vehicle structure of which a multi-ratio transmission device is installed at the rotation center of the crank.
2. The Prior Arts
In order for two-wheel vehicles to be ridden easily in different road conditions or different environmental conditions, two-wheel vehicles, e.g. bicycle, electric bicycle, on the market nowadays are usually equipped with multi-ratio transmission systems, so the rider can adjust gear ratios according to the conditions of the road, thereby riding the vehicle with less effort.
Bicycle derailleur is one of the commonly seen multi-ratio transmission systems, which mainly consists of different sizes of sprockets. As shown in FIG. 1, a conventional bicycle derailleur system 900 includes multiple sprockets 902 which are coaxially disposed on the bicycle along bicycle wheel axle 904, e.g. the rear bicycle wheel axis. The sprockets 902 are connected to the bicycle pedals through a chain 906. In the system as described above, derailing the chain 906 between sprockets 902 of different sizes can change the rotation speed ratio between the rear bicycle wheel and the pedal. Due to the nature of the structure of the derailleur, the axial size of the derailleur system would increase as more sprockets are added to the system. Therefore, only a limited number of sprockets can be used in such derailleur system to prevent the size of the system from becoming too large and thus affecting the structure of the bicycle. To be more specific, if the size of the derailleur becomes too large, the bicycle frame can become eccentric and the chain of the transmission system can be dislocated during the derailing, thereby resulting in the loss of kinetic energy during transmission and further affecting the operation of the bicycle. However, since each sprocket represents a different rotation speed, the number of the sprockets determines the number of transmission ratios available in a bicycle derailleur system. Hence, under the condition that the number of sprockets is limited, the number of the transmission ratios that can be provided by the derailleur is also limited.
Another type of multi-ratio transmission system commonly seen on bicycles is the multi-ratio hub, which utilizes planet gear systems to achieve the multi-ratio effect. Planet gear system is an effective way to reduce the size of the gear transmission system in the mechanical industry. FIG. 2 is an example of a commonly seen planet gear system. As shown in FIG. 2, the planet gear system 940 includes a sun gear 942 and an annular gear 944. The sun gear 942 and the annular gear 944 are coaxially disposed to form an annular space within. Multiple planet gears 946 are placed inside the annular space to simultaneously engage with the sun gear 942 and the annular gear 944. With such configuration, the sun gear 942, the annular gear 944 and the planet gears 946 basically rotate in different speed. When the planet gear system as described above is in use, each of the sun gear, the planet gear and the annular gear serve as the input end or the output end to change the rotation speed and the torque between the input end and the output end. However, the number of transmission ratio that can be achieved by a single planet gear system is limited; hence, two sets or more of the planet gear systems are often coupled together in the axial direction to increase the number of the transmission ratio or torque available. In addition, the rotation axes of the planet gears are parallel to the rotation axis of the sun gear, which can still result in the increase of the overall size of the system, the abrasion of the gear due to the direct force exerted thereupon and the loss of mechanical kinematic energy.
In order to solve the abovementioned problems, the applicant has invented a multi-ratio transmission system with parallel vertical and coaxial planet gears, and was filed as TW patent application No. 101120752, 101120748, 101120934, 101120938, 101120940, 101120943. The multi-ratio transmission system with parallel vertical and coaxial planet gears not only provides a significant number of gear ratios available, but also greatly reduced the weight and over all size of the multi-ratio transmission system. Compared with other conventional transmission systems, such multi-ratio transmission system is more suitable to be installed on a bicycle. Due to the limit in size, conventional multi-ratio transmission systems are all installed on the wheel axle of the rear wheel in conventional two-wheel structures. When the rider pushes the pedals, the rotation motion is transmitted to the transmission system via a chain so as to propel the rear wheel. In the configuration of the conventional transmission system on the conventional vehicle structure as described above, work done by the rider is not directly exerted into the transmission system; therefore, the kinetic energy generated by the rider by pushing the pedals to rotate certainly will suffer some loss when it is transmitted to the transmission system installed at the rear wheel via the chain. In addition, other factors such as the dislocation of the chain during gear-shifting can cause more energy loss, thereby resulting in a waste of the output power generated by the rider. Furthermore, in addition to the multi-ratio transmission system, the wheel axle of the rear wheel also has to bear the weight of the rider and the frame, thus the axle of the transmission system can be easily damaged. Moreover, the size of transmission system in conventional bicycle usually varies corresponding to the number of gear ratio being provided; hence, the frame structure also needs to be adjusted according to the size of the transmission system. In order to install transmission system with a larger size, the bicycle frame may have an eccentric structure, which can cause difficulty in mass manufacture and application thereof.
Therefore, it is urgently needed for the industry to develop a two-wheel vehicle structure, which can provide a large number of transmission ratios and a better output efficiency while reducing the energy loss in the transmission system. In addition, the two-wheel vehicle structure should also lower the failure rate of the transmission system, provide a more convenient way of usage and make the mass manufacture process of the frame easier.